Control apparatus and control method of fuel cell system

ABSTRACT

A control apparatus and a control method are used in a fuel cell system, in which fuel gas and oxidant gas are supplied to a fuel cell stack  1  to generate electric power, a load unit  9  is driven by supplying the generated power thereto, and the generated power is stored in a power storage unit. A control unit  10  sets in advance a margin load power to be supplied from a power storage unit  8  to the fuel cell stack  1  when the load of the load unit  9  changes at a predetermined rate, and computes outputable power of the power storage unit  8 . Then, the control unit  10  compares the margin load power and the outputable power to generate an amount judgment result. When the outputable power is larger than the margin load power, the control unit  10  controls the power generation amount of the fuel cell stack  1  such that charged power becomes equal to an electric power difference between the margin load power and the outputable power.

TECHNICAL FIELD

[0001] The present invention relates to a control apparatus and acontrol method of a fuel cell system, in which fuel gas and oxidant gasare supplied to a fuel cell to generate electric power, the generatedpower is supplied to a load circuit to drive a load, and the generatedpower is also stored in a power storage unit.

BACKGROUND ART

[0002] In a fuel cell system used as a power source of a vehicle, a fuelcell stack is often used, which is obtained in such a manner that a fuelcell structure having an oxidant electrode and a fuel electrode providedas a pair interposing a solid polymer electrolyte film therebetween issandwiched by separators, and a plurality of the fuel cell structuresare laminated on each other. Electric power generated from the fuel cellstack is charged to a battery and then used to drive a motor in thevehicle.

[0003] With respect to a fuel cell system, a control performed in such amanner that a charge level of a battery (SOC) is set high at the time oflow speed driving to secure driving force of a motor whereby to secureelectric power capable of being outputted from the battery, is disclosedin the gazette of Japanese Patent Laid-Open No. 2000-92610.

[0004] Also, with respect to a conventional fuel cell system, forexample, a method is disclosed in the gazette of Japanese PatentLaid-Open No. 10(1998)-271706. In this method, detected is a state wherea main load such as a motor installed in a vehicle is almost zero, andas a result of the detection, when a judgment for the detected statetells that a battery is to be charged, the battery is charged. Accordingto this method, in the conventional fuel cell system, it has beenpossible to supply sufficient electric power at all time even when theelectric power required by the load changes largely.

DISCLOSURE OF INVENTION

[0005] However, in a fuel cell system disclosed in the gazette ofJapanese Patent Laid-Open No. 2000-92610, a control is performed in sucha manner that a vehicle speed is detected and a target charge level isset in accordance with the detected vehicle speed, thus securingoutputable power of a battery. Therefore, it requires a certain amountof time for actually charging the battery and securing the output,resulting in occurrence of a problem that the fuel cell system cannotmeet immediate needs in some cases.

[0006] Also, in a fuel cell system disclosed in the gazette of JapanesePatent Laid-Open No. 10(1998)-271706, a control is performed in such amanner that a state, in which a battery should be charged is consideredas a state in which a main load becomes almost zero, is detected, then abattery is judged to be charged. Therefore, when a relativelyintermediate load continues, that is, when a period of time when themain load is almost zero is short, the battery cannot be sufficientlycharged. Consequently, in this fuel cell system, there has been aproblem that supplied electric power to a change in used power requiredby the load becomes deficient.

[0007] The present invention was made with taking the above-describedproblems into consideration, and an object thereof is to provide acontrol method and a control apparatus for a fuel cell system capable ofachieving sufficient power supply at any time to a load whose used powerchanges.

[0008] In order to solve the above-described problems, a first aspect ofthe present invention is a control apparatus for a fuel cell system, inwhich fuel gas and oxidant gas are supplied to a fuel cell to generateelectric power, a load is driven by supplying the generated power to aload circuit, and the generated power from the fuel cell is stored in apower storage unit. The control apparatus comprises: a margin load powersetting section for setting a margin load power amount, which is anincreased amount of the electric power supplied from the power storageunit and the fuel cell to the load circuit when the load of the loadcircuit is increased at a predetermined rate; an outputable powercomputing section for computing outputable power of the power storageunit; an inputable power computing section for computing electric powercapable of being inputted and stored in the power storage unit; and acontrol section for comparing the margin load power set by the marginload power setting section and the outputable power computed by theoutputable power computing section to generate an amount judgmentresult, comparing the inputable power and an electric power differencebetween the margin load power and the outputable power when theoutputable power is smaller than the margin load power as a result ofthe amount judgment to generate an amount judgment result, generatingelectric power to set the inputable power to be charged power of thepower storage unit when an electric power difference between the marginload power and the outputable power is larger than the inputable poweras a result of the amount judgment, and controlling a flow rate orpressure of gas supplied to the fuel cell to make it possible to alwaysgenerate electric power equivalent to a deficient amount of the electricpower obtained by subtracting the inputable power and the outputablepower from the margin load power.

[0009] Furthermore, a second aspect of the present invention is acontrol method for a fuel cell system, in which fuel gas and oxidant gasare supplied to a fuel cell to generate electric power, a load is drivenby supplying the generated power to a load circuit, and the generatedpower from the fuel cell is stored in a power storage unit. The controlmethod comprises the steps of: setting in advance a margin load poweramount, which is an increased amount of an electric power amountsupplied from the power storage unit and the fuel cell to the load whenthe load of the load circuit is increased at a predetermined rate;computing electric power capable of being inputted and stored in thepower storage unit, and computing outputable power of the power storageunit; generating an amount judgment result by comparing the inputablepower and the electric power difference between the margin load powerand the outputable power; and generating the electric power such thatthe inputable power becomes the charged power to the power storage unit,and controlling the flow rate or the pressure of the gas supplied to thefuel cell such that a deficient amount of the electric power obtained bysubtracting the inputable power and the outputable power from the marginload power is always generated when the electric power differencebetween the margin load power and the outputable power is larger thanthe inputable power as a result of the amount judgment.

BRIEF DESCRIPTION OF DRAWINGS

[0010]FIG. 1 is a constitutional diagram of a fuel cell system to whichthe present invention is applied.

[0011]FIG. 2 is a block diagram showing a functional constitution of acontrol unit provided in the fuel cell system, to which the presentinvention is applied.

[0012]FIG. 3 is a graph showing a relation between electric powerrequired by a load unit and margin load power.

[0013]FIG. 4 is a graph showing a relation between output power of afuel cell stack, and pressure values and flow rates of fuel gas andoxidant gas.

[0014]FIG. 5 is a graph showing a relation among a charge level,inputable power and outputable power.

[0015]FIG. 6 is a flow chart showing process steps of power controlprocess by the control unit.

[0016]FIG. 7 is a flow chart showing process steps of power controlprocess by the control unit.

[0017]FIG. 8A shows a relation among a load required by the load unit, atarget power generated amount and outputable power against margin loadpower, FIG. 8B shows a relation among a load required by the load unit,a target power generated amount against margin load power, outputablepower and a deficient amount, and FIG. 8C shows a relation among a loadrequired by the load unit, a target power generated amount againstmargin load power, outputable power, target FC margin power and adeficient amount.

[0018]FIG. 9A is a graph showing a relation between output of the fuelcell stack and a load required by the load unit, and FIG. 9B is a graphshowing a change in electric power in the power storage unit.

[0019]FIG. 10A is another graph showing a relation between output of thefuel cell stack and a load required by the load unit, and FIG. 10B isanother graph showing a change in electric power in the power storageunit.

[0020]FIG. 11 is a flow chart of a notification process performed for auser when target FC margin power is supplied to the load unit by thecontrol unit.

[0021]FIG. 12 is a flow chart showing process steps for judging whetheror not electric power equivalent to target FC margin power can begenerated, based on the temperature of the fuel cell stack.

[0022]FIG. 13 is a graph showing a relation between a coolanttemperature (fuel cell temperature) from the fuel cell stack and maximumoutputable power of the fuel cell stack.

[0023]FIG. 14 is a flow chart showing process steps for judging whetheror not electric power equivalent to target FC margin power can begenerated, based on a stoichiometric ratio of fuel gas or oxidant gas.

[0024]FIG. 15 is a graph showing a relation between the stoichiometricratio and electric power generated from the fuel cell stack.

BEST MODE FOR CARRYING OUT THE INVENTION

[0025] Hereinafter, an embodiment of the present invention will bedescribed with reference to the drawings.

[0026] The present invention is applied to a fuel cell systemconstituted as shown in FIG. 1 for example.

[0027] This fuel cell system is provided with a fuel cell stack 1 whichis supplied with hydrogen-containing gas and fuel gas to generateelectric power. This fuel cell stack 1 consists of, for example, aplurality of fuel cell structures in which separators sandwich a fuelcell structure mounting a pair of an oxidant electrode and a fuelelectrode that are interposing a solid polymer electrolyte filmtherebetween. This fuel cell stack 1 generates electric power by beingsupplied with oxygen-containing air as oxidant gas to its oxidantelectrode and by being supplied with hydrogen gas as fuel gas to itsfuel electrode, and is utilized as a drive source of, for example, avehicle or the like.

[0028] Constitution of Fuel Cell System

[0029] As apparent from FIG. 1, this fuel cell system is provided with afuel tank 2 for storing hydrogen gas to be supplied to the fuel cellstack 1 and a pressure control valve 3 for adjusting pressure and a flowrate of fuel gas to be supplied to the fuel cell stack 1. Here, the fuelcell stack 1, the fuel tank 2 and the pressure control valve 3 areconnected to each other via a hydrogen passage tube. In the pressurecontrol valve 3, valve opening thereof is adjusted according to anopening control signal S1 from a later-described control unit 10, andthus the pressure and the flow rate of the fuel gas to be supplied tothe fuel cell stack 1 are controlled.

[0030] Also, this fuel cell system is provided with a compressor 4 forsupplying the oxidant gas to the fuel cell stack 1, a motor 5 fordriving the compressor 4 and a pressure control valve 6 disposed on anoxidant gas exhausting side of the fuel cell stack 1. Here, thecompressor 4, the fuel cell stack 1, and the pressure control valve 6are connected to each other via an oxidant gas passage tube. The controlunit 10 controls driving force of the compressor 4 based on drivingforce of the motor 5 by outputting a driving force control signal S2 tothe motor 5. The control unit 10 also controls oxidant gas pressure inthe fuel cell stack 1 by controlling the pressure control valve 6 byoutputting a valve opening control signal S3 thereto, wherebycontrolling the flow rate of the oxidant gas to be supplied to the fuelcell stack 1.

[0031] Moreover, this fuel cell system is provided with a power controlunit 7 electrically connected to the fuel cell stack 1, a power storageunit 8 for storing electric power generated by the fuel cell stack 1 anda load unit 9 such as a drive motor, which is driven by the electricpower generated by the fuel cell stack 1. The storage unit 8 is composedof so-called batteries.

[0032] The power control unit 7 takes out the electric power from thefuel cell stack 1 according to a power control signal S4 from thecontrol unit 10 and supplies the electric power to the motor 5 of thecompressor, the load unit 9, the power storage unit 8 and the like. Whenan amount of the electric power supplied from the power control unit 7is larger than that consumed in the load unit 9, the power storage unit8 stores surplus electric power from the fuel cell stack 1 as chargedpower. When the amount of electric power supplied from the power controlunit 7 is smaller than that consumed in the load unit 9, the powerstorage unit 8 supplies the electric power equivalent to the deficientamount to the load unit 9.

[0033] The control unit 10 computes accelerator operating amountinformation operated by a vehicle driver and vehicle speed informationbased on a vehicle speed pulse signal. The control unit 10 controlsdriving force by outputting a driving force control signal S5 to theload unit 9 according to magnitude indicated by the acceleratoroperating amount information. Moreover, the control unit 10 obtains atarget power generation amount through computation based on theaccelerator operating amount information and the vehicle speedinformation.

[0034] The control unit 10 computes the target power generation amountserving as a target of electric power generated from the fuel cell stack1. Also, the control unit 10 controls the valve opening of the pressurecontrol valve 3, the driving force of the motor 5 and the valve openingof the pressure control valve 6 so as to obtain a desired powergeneration amount, and controls the pressure and the flow rate of thefuel gas and the oxidant gas supplied to the fuel cell stack 1, thuscontrolling the power generation amount of the fuel cell stack 1.Furthermore, the control unit 10 controls the power control unit 7, thussupplying electric power to the power storage unit 8 and the load unit9.

[0035] Functional Constitution of Control Unit 10

[0036]FIG. 2 is a block diagram showing a functional constitution of thecontrol unit 10. As apparent from FIG. 2, the control unit 10 includes arequired load computing section 21 to which the accelerator operatingamount information and the vehicle speed information are inputted, aninputable/outputable power computing section 22 to which batterytemperature information from a battery sensor for detecting a state ofthe power storage unit 8 (not shown) and charging level informationshowing a charging level of the power storage unit 8 are inputted, and amargin load power memory section 23 with a value of electric powerstored, which is to compensate the electric power required to the loadunit 9 at the time of the maximum acceleration of the vehicle.

[0037] Moreover, the control unit 10 includes a target charge amountcomputing unit 24 for computing by the use of the pieces of informationfrom the inputable/outputable power computing section 22 and the marginload power memory section 23, a target FC margin power computing section25, an auxiliary device consumption power computing section 26 forcomputing consumed electric power in each component constituting thefuel cell system excluding the load unit 9, a target power generationamount computing section 27 for computing by the use of the pieces ofinformation from the required load computing section 21, the targetcharge amount computing unit 24 and the auxiliary device consumptionpower computing section 26. The control unit 10 further includes a gassupply amount control section 28 for computing by the use of the piecesof information from the target power generation amount computing section27 and the target FC margin power computing section 25.

[0038] The required load computing section 21 computes required loadpower required to drive the load unit 9, based on the acceleratoroperating amount information and the vehicle speed information. Then,the required load computing section 21 outputs computed required loadpower information S11 to the target power generation amount computingsection 27.

[0039] The inputable/outputable power computing section 22 computesinputable power information showing a value of the electric powercapable of being stored in the power storage unit 8 and outputable powerinformation showing a value of the electric power capable of beingoutputted from the power storage unit 8 to the load unit 9, based on thebattery temperature and the charging level information. Then, theinputable/outputable power computing section 22 outputs inputable powerinformation S14 and outputable power information S13 to the targetcharge amount computing unit 24 and the target FC margin power computingsection 25.

[0040] The target charge amount computing unit 24 computes a targetcharge amount serving as a target of electric power which is generatedby the fuel cell stack 1 and which is also stored in the power storageunit 8 based on margin load power information S12, the inputable powerinformation S14 and the outputable power information S13. Then, thetarget charge amount computing unit 24 determines the target chargeamount within a range of the inputable power so as to allow the chargedpower to approach the margin load power and then outputs target chargedpower information S16 to the target power generation amount computingsection 27.

[0041] The target FC margin power computing section 25 computes based onthe margin load power information S12, the outputable power informationS13 and the inputable power information S14. Then, the target FC marginpower computing section 25 obtains target FC margin power informationS17 showing the target FC margin power capable of being taken outimmediately and supplied to the load unit 9, and outputs the same to thegas supply amount control section 28.

[0042] The target power generation amount computing section 27 computesbased on auxiliary device consumption power information S15, the targetgenerated power information S16 and the required load power informationS11. Then, the target power generation amount computing section 27obtains target generated power information S18 showing a value of theelectric power serving as a target of the electric power generated bythe fuel cell stack 1 and outputs the same to the gas supply amountcontrol section 28.

[0043] The gas supply amount control section 28 controls the pressurecontrol valve 3, the motor 5 and the pressure control valve 6 based onthe target generated power information S18 and the target FC marginpower information S17 to control the pressure and the flow rate of thefuel gas and the oxidant gas supplied to the fuel cell stack 1, thuscontrolling the generated power of the fuel cell stack 1.

[0044]FIG. 3 shows changes in electric power supplied from the fuel cellstack and the power storage unit 8 to the load so as to correspond tothe changes in the electric power required by the load unit 9. Asapparent from FIG. 3, in the case where the curve A represents theelectric power necessary for driving the load unit 9, if the fuel cellstack 1 can only generate the electric power represented by the curve B,the power generation amount of the fuel cell stack 1 is compensated bythe power storage unit 8 supplying the electric power to the load unit9. Specifically, the electric power represented by the curve C is forcompensating the electric power represented by the shaded area in FIG.3. Here, assuming the electric power represented by the curve C requiredat the time of the maximum acceleration of the vehicle, the value of themargin load power is set in advance and is stored in the margin loadpower memory section 23.

[0045]FIG. 4 shows a relation between output power of the fuel cellstack 1 and pressure values as well as flow rates of fuel gas andoxidant gas. In FIG. 4, solid lines represent the minimum necessary flowrate and the minimum necessary pressure for taking out the electricpower from the fuel cell stack 1. Here, the gas supply amount controlsection 28 controls each component such that the control target flowrate and pressure are set as the flow rate value and the pressure value(shown by  in FIG. 4) increased by the target FC margin power, andthen, the electric power actually taken out from the fuel cell stack 1is set as the target power generation amount (shown by ∘ in FIG. 4).Consequently, a sum of the electric power of the target power generationamount and the target FC margin power can be taken out from the fuelcell stack 1. When the load unit 9 is operated at the maximum load andthe requirement for the required load power occurs, the control unit 10controls such that the amount of the electric power equivalent to thetarget FC margin power is immediately taken out from the fuel cell stack1 and is supplied to the load unit 9.

[0046] On the other hand, when the target FC margin power is zero, thegas supply amount control section 28 controls the motor 5 to reduce adrive amount of the compressor 4 so as to obtain the minimum necessaryflow rate and the minimum necessary pressure, and then, the electricpower actually taken out from the fuel cell stack 1 is set as the targetpower generation amount. In other words, when obtaining the output powerrepresented by ∘ in FIG. 4, the gas supply amount control section 28sets the points, at which the dotted line extending from ∘ in FIG. 4crosses with the lines representing each of properties, as the controltarget flow rate and the control target pressure.

[0047]FIG. 5 shows changes in the outputable power and the inputablepower with respect to a charge level (SOC). The outputable power and theinputable power change also with temperatures, and FIG. 5 shows each ofthe properties at a room temperature and at a low temperature.

[0048] Power Control Process of Control Unit 10

[0049] In FIGS. 6 and 7, process steps of a power control process by thecontrol unit 10 having the above-described constitution are shown.According to FIG. 6, when driving the load unit 9, the control unit 10first detects a temperature of the power storage unit 8 by theoutputable power computing section 22 based on a sensor signal from thebattery sensor (not shown) in step ST1, and then the process proceeds tothe next step ST2.

[0050] In step ST2, the control unit 10 calculates the charge levelbased on a current value and a voltage value of the power storage unit 8and outputs the result to the inputable/outputable power computingsection 22.

[0051] Next, in step ST3, the inputable/outputable power computingsection 22 calculates the inputable power information S14 and theoutputable power information S13 according to the battery temperatureand the charge level obtained in steps ST1 and ST2. Here, theinputable/outputable power computing section 22 obtains the inputablepower and the outputable power based on the properties of the inputablepower and the outputable power with respect to the charge level as shownin FIG. 5, the battery temperature and the charge level.

[0052] Subsequently, the control unit 10 reads out the margin load powerinformation stored in the margin load power memory section 23 in stepST4.

[0053] In the next step ST5, the accelerator operating amountinformation and the vehicle speed information are inputted to therequired load computing section 21. In the next step ST6, the requiredload computing section 21 computes the required load power required bythe load unit 9 based on the accelerator operating amount informationand the vehicle speed information.

[0054] In the next step ST7, the auxiliary device consumption powercomputing section 26 computes the auxiliary device consumption powerinformation S15.

[0055] In the next step ST8 shown in FIG. 7, the target charge amountcomputing unit 24 compares the amount of the margin load power stored inthe margin load power memory section 23 and the amount of the outputablepower obtained in step ST3 to determine whether or not the margin loadpower is smaller than the outputable power. When the margin load poweris smaller than the outputable power, the target charge amount computingunit 24 proceeds to step ST9, and proceeds to step ST11 described laterwhen the margin load power is not smaller than the outputable power.

[0056] When the margin load power is smaller than the outputable power,the target charge amount computing unit 24 recognizes that theoutputable power can cover the margin load power as shown in FIG. 8A andthat the margin load power can be secured without additional charging.Then, the target charge amount computing unit 24 outputs the targetcharged power information S16 having a zero target charge amount to thetarget power generation amount computing section 27, and then proceedsto step ST9.

[0057] In step ST9, the target power generation amount computing section27 takes the target power generation amount to be equal to the requiredload power obtained in step ST6, and outputs the target generated powerinformation S18 to the gas supply amount control section 28. Then, thetarget FC margin power is set 0 in step ST19. Thereafter, the processproceeds to step ST10. Here, the target power generation computingsection 27 includes also the auxiliary device consumption power,obtained in step ST7, in the target power generation amount.

[0058] On the other hand, if the margin load power is determined notsmaller than the outputable power in step ST8, the target charge amountcomputing unit 24 computes an electric power difference of theoutputable power with respect to the margin load power in step S11. Inthis case, the target charge amount computing unit 24 recognizes theelectric power difference by subtracting outputable power from themargin load power.

[0059] Subsequently, in step ST12, the target charge amount computingunit 24 determines whether or not the electric power difference computedin step ST11 is smaller than the inputable power. When it is determinedthat the power difference is smaller than the inputable power, thetarget charge amount computing unit 24 proceeds to step ST13, but whendetermined that the electric power difference is not smaller than theinputable power, the target charge amount computing unit 24 proceeds tostep ST15 described later.

[0060] In step ST13, the target charge amount computing unit 24recognizes that the electric power difference can be covered by chargingthe electric power equivalent to the inputable power as shown in FIG.8B, and sets the electric power difference computed in step ST11 as thetarget charge amount as it is.

[0061] In the next step ST14, the target power generation amountcomputing section 27 receives the required load power obtained in stepST6 from the required load computing section 21, and at the same time,receives the target charge amount obtained in step ST13 from the targetcharge amount computing unit 24. Then, the electric power amountobtained by adding the required load power and the target charge amountis set as the target power generation amount, and the target FC marginpower is set 0 in step ST18. Thereafter, the process proceeds to stepST10.

[0062] If it is determined that the electric power difference is notsmaller than the inputable power in step ST12, the target charge amountcomputing unit 24 recognizes in step ST15 that the electric powerdifference cannot be covered by the inputable power as shown in FIG. 8Cand sets the inputable power as the target charge amount.

[0063] In the next step ST 16, the target power generation amountcomputing section 27 adds the required load power from the required loadcomputing section 21 and the target charge amount to obtain the targetpower generation amount. Here, the target power generation amountcomputing section 27 includes the auxiliary device consumption power,obtained in the above-described step ST7, in the target power generationamount.

[0064] In the next step ST17, the target FC margin power computingsection 25 obtains the electric power difference by subtracting theoutputable power from the margin load power, and then, the target FCmargin power (deficient amount of the electric power) is obtained bysubtracting the inputable power from the obtained power difference.Thereafter, the process proceeds to step ST10. In the next step ST10,the gas supply amount control section 28 outputs the control signal forcontrolling each component based on the target generated powerinformation S18, and then sets the flow rate and the pressure of thefuel gas and the oxidant gas so as to obtain the target generationamount and the target FC margin power as shown in FIG. 4.

[0065] Effect of the Embodiment

[0066] According to the control unit 10 performing the process asdescribed above, in the case where the outputable power is smaller thanthe margin load power as shown in FIG. 8B, and also when the inputablepower is larger than the electric power difference, the target powergeneration amount is set so as to compensate the electric powerdifference with the target charge amount. By doing so, the margin loadpower can be taken out at any time.

[0067] As shown in FIG. 9A for example, the generated power output ofthe fuel cell stack 1 exceeds the required load during the period T1before the load of the load unit 9 is changed, and the electric power ischarged to the power storage unit 8 as shown in FIG. 9B. The sum totalof the target charged power at this time and the outputable powerbecomes the margin load power. Note that the difference between the fuelcell output (Net) and the fuel cell output (Gross) is the electric powerconsumed by the auxiliary devices.

[0068] Here, as shown in FIG. 9A, the electric power equivalent to thetarget charge amount can be supplied since the target charge amount isset so as to secure the margin load power at all times even in the casewhere the required load is largely changed during the next period T2,the change in the electric power output from the fuel cell stack 1delays with respect to the changing speed of the required load, andfurthermore, even in the case where the required load exceeds the sumtotal value of the electric power output from the fuel cell stack 1 andthe outputable power of the power storage unit 8. Therefore, accordingto the fuel cell system, even when the load of the load unit 9 isdrastically changed, it becomes possible to supply the margin load powerto the load unit 9 and to make the electric power supply to follow thechange in the required load.

[0069] Also, in this fuel cell system, the electric power supplied fromthe power storage unit 8 to the load unit 9 can be kept within theoutputable power. Therefore, according to the fuel cell system, itbecomes possible to inhibit the progression of deterioration of thepower storage unit 8.

[0070] Moreover, during the period T3 after the load change, the fuelcell system continues to charge the power storage unit 8, thus making itpossible to set the charge level high enough to cover the margin loadpower with the outputable power. Therefore, according to the fuel cellsystem, when the required load of the load unit 9 is not changed,electric charge is performed to the power storage unit 8, thus making itpossible to supply the margin load power.

[0071] Also, according to the control unit 10 performing the process asdescribed above, in the case where the outputable power is smaller thanthe margin load power as shown in FIG. 8C, even when the inputable poweris smaller than the electric power difference therebetween, the marginload power can be secured by setting the target FC margin power.

[0072] For example, as shown in FIG. 10A, during the period T1 beforethe load of the load unit 9 changes, the fuel cell output (Net) exceedsthe required load, and the electric power is charged to the powerstorage unit 8 with the electric power smaller than the inputable poweras shown in FIG. 10B.

[0073] Here, it is assumed that, since the inputable power of the powerstorage unit 8 is small during the period T1, the margin load powercannot be covered only by the sum total of the charged electric powerand the outputable power in the period T1. Contrary to this, in the fuelcell system, it is assumed that the target FC margin power is set instep ST17 and the surplus electric power can always be outputted. Then,when the required load of the load unit 9 becomes large, the electricpower already taken out and the target FC margin power are taken outfrom the fuel cell stack 1, and at the same time, the charged power ofthe power storage unit 8 is supplied to the load unit 9.

[0074] Therefore, according to the fuel cell system, it is possible toimmediately take out the target FC margin power and to shorten theperiod of time in which the output of the fuel cell stack 1 isinsufficient with respect to the load, and it is also possible to reducethe insufficient amount of the output of the fuel cell stack 1. Also,the electric power can be supplied in response to the change in therequired load.

[0075] Furthermore, according to the fuel cell system, it is possible tocope with the load change rapidly by supplying the target FC marginpower to the load unit 9 even when the power storage unit 8 at a lowtemperature cannot output much electric power in comparison to that atthe room temperature as shown in FIG. 5, and even when electric power isnot thus sufficiently supplied with respect to the load change.

[0076] Also, according to the fuel cell system, the electric powerinputted to the power storage unit 8 can be kept within the outputablepower, and the electric power outputted from the power storage unit 8can be kept within the outputable power. Therefore, it is possible toinhibit the progression of deterioration of the power storage unit 8.

[0077] Notification Process of the Control Unit 10

[0078] Next, description will be made for a notification process for auser at the time of supplying the target FC margin power to the loadunit 9 by the control unit 10 with reference to FIG. 11.

[0079] According to FIG. 11, first in step ST21, the target FC marginpower is computed by the target FC margin power computing section 25similarly to the above-described step ST17 for example, and the processproceeds to step ST22.

[0080] In step ST22, the target FC margin power computing section 25controls the pressure control valve 3, the motor 5 and the pressurecontrol valve 6, and then judges whether or not the target FC marginpower computed in step ST21 can be obtained. When it is judged possibleto obtain the target FC margin power, the target FC margin powercomputing section 25 proceeds to step ST23. When it is judged impossibleto obtain the target FC margin power, the target FC margin powercomputing section 25 proceeds to step ST24.

[0081] In step ST23, the target FC margin power computing section 25carries out a control so as to turn off a margin deficiency lamp, whichnotifies the vehicle driver that it is impossible to supply the targetFC margin power.

[0082] In step ST24, the target FC margin power computing section 25carries out a control so as to turn on the margin deficiency lamp.

[0083] By doing so, in the fuel cell system, it becomes possible tonotify the vehicle driver that the acceleration performance is lowered.In addition, by notifying that there remains little power left for ademanding case such as rapid acceleration, it becomes possible topromote driving operations according to situations.

[0084] Here, means for notifying infeasibility of the target FC marginpower to the vehicle driver is not limited to a lamp, and a displaymachinery and an audio machinery (not shown) may be used.

[0085] Judgment Process Based on Temperature of Fuel Cell Stack 1

[0086] Next, description will be made for an example with reference toFIG. 12, in which the judgment in step ST22 shown in FIG. 11 is carriedout based on a temperature of the fuel cell stack 1.

[0087] According to FIG. 12, in step ST31, the target FC margin powercomputing section 25 computes the target FC margin power similarly tothe above-described process and starts the following process.

[0088] In the next step ST32, a temperature of a coolant supplied to thefuel cell stack 1 is detected by a temperature sensor (not shown).

[0089] In the next step ST33, the maximum possible power generationamount of the fuel cell stack 1 is computed based on the coolanttemperature detected in step ST32. Here, the maximum possible powergeneration amount of the fuel cell stack 1 is obtained based on thecoolant temperature with reference to the relation between the coolanttemperature (fuel cell temperature) from the fuel cell stack 1 and themaximum possible power generation amount of the fuel cell stack 1 asshown in FIG. 13.

[0090] In the next step ST34, a sum total amount of the target generatedpower and the target FC margin power is compared to the maximum possiblepower generation amount of the fuel cell stack 1. When the sum totalamount of the target generated power and the target FC margin power issmaller than the maximum possible power generation amount of the fuelcell stack 1, the process proceeds to step ST35. When the sum totalamount of the target generated power and the target FC margin power isnot smaller than the maximum possible power generation amount of thefuel cell stack 1, the process proceeds to step ST37.

[0091] In step ST35, the flow rate and the pressure of the fuel gas andthe oxidant gas for generating the electric power are computed, wherethe electric power is the sum total of the target generated power andthe target FC margin power at the fuel cell stack 1. Then the processproceeds to step ST36.

[0092] In step ST36, it is judged that the target FC margin power isfeasible, and then the process proceeds to step ST 23 in FIG. 11. Then,the process proceeds to step ST23 in FIG. 11.

[0093] On the other hand, when it is judged that the sum total amount ofthe target generated power and the target FC margin power is larger thanthe maximum possible power generation amount of the fuel cell stack 1 instep ST34, the process proceeds to step ST37. In step ST37, the gassupply amount control section 28 computes the flow rate and the pressureof the fuel gas and the oxidant gas based on the maximum possible powergeneration amount of the fuel cell stack 1 computed in step ST33. Then,the process proceeds to step ST38.

[0094] In step ST38, it is judged that the target FC margin power isinfeasible. Then, the process proceeds to step ST24 in FIG. 11.

[0095] According to the fuel cell system performing such process asdescribed above, the maximum possible power generation amount of thefuel cell stack 1 which changes with the temperature changes of the fuelcell stack 1 is obtained to judge whether or not the target FC marginpower can be supplied in order to cover the margin load power.Therefore, it becomes possible to accurately determine that thereremains little power left for an excessive load change based on thetemperature state of the fuel cell stack 1.

[0096] Judgment Process Based on Stoichiometric Ratio

[0097] Next, description will be made for an example where the judgmentof step ST22 in FIG. 11 is carried out based on the stoichiometric ratioof the fuel gas or the oxidant gas shown in FIG. 14.

[0098] As shown in FIG. 14, in step ST41, the target FC margin power iscomputed by the target FC margin power computing section 25 similarly tothe above-described process. Then the process proceeds to step ST42 tostart the following process.

[0099] In step ST42, a limit stoichiometric ratio is computed based onthe target power generation amount and the target FC margin power. Thenthe process proceeds to step ST43. Here, the stoichiometric ratio is avalue obtained by dividing the gas supply amount to the fuel cell stack1 by the gas supply amount consumed by fuel cell stack 1, and thestoichiometric ratio changes depending on the generated power of thefuel cell stack 1 as shown in FIG. 15. In this case, the limitstoichiometric ratio changes also depending on pure-water collectionefficiency and humidification properties of the fuel cell system.

[0100] In step ST43, a target FC margin power amount implementingstoichiometric ratio is computed by dividing the flow rates of the fuelgas and the oxidant gas necessary for implementing the power generationof the sum total of the target generated power and the target FC marginpower by the flow rates of the fuel gas and the oxidant gas consumedwhen the power is generated. Then, the process proceeds to step ST44.

[0101] In step ST44, the limit stoichiometric ratio computed in stepST42 and the target FC margin power amount implementing stoichiometricratio computed in step ST43 are compared to each other. When it isjudged that the target FC margin power amount implementingstoichiometric ratio is larger than the limit stoichiometric ratio, theprocess proceeds to step ST45. When it is judged that the target FCmargin power amount implementing stoichiometric ratio is not larger thanthe limit stoichiometric ratio, the process proceeds to step ST47.

[0102] In step ST45, a target stoichiometric ratio is set as the targetFC margin power amount implementing stoichiometric ratio, and then theprocess proceeds to step ST46.

[0103] In step ST46, it is judged that the target FC margin power isfeasible, and then the process proceeds to step ST 23 in FIG. 11.

[0104] In step ST47, a target stoichiometric ratio is set as the limitstoichiometric ratio shown in FIG. 15, and then the process proceeds tostep ST48.

[0105] In step ST48, it is judged that the target FC margin power isinfeasible, and then the process proceeds to step ST24 in FIG. 11.

[0106] According to the fuel cell system performing such process asdescribed above, the maximum possible power generation amount of thefuel cell stack 1 which changes depending on the consumption change ofthe fuel cell stack 1 is obtained, and it is then judged whether or notthe target FC margin power for covering the margin load power can besupplied. Therefore, it becomes possible to accurately determine thatthere remains little power left for an excessive load change based onthe consumption state of the fuel cell stack 1.

[0107] Note that the above-described embodiment is an example of thepresent invention. Accordingly, the present invention is not limited tothe above-described embodiment. As a matter of course, even in a casedifferent from the above-described embodiment, various modifications arepossible depending on the design and the like within the scope of thetechnical concepts related to the present invention.

INDUSTRIAL APPLICABILITY

[0108] According to the control apparatus of the fuel cell system of thepresent invention, when the outputable power is smaller than the marginload power, the power generation amount of the fuel cell is controlledsuch that the charged power becomes larger than the electric powerdifference between the margin load power and the outputable power.Therefore, even if the load of the load circuit is increased, theelectric power is supplied from the power storage unit, and the electricpower equivalent to the margin load power can be supplied. Thus, it ispossible to realize sufficient power supply to the load whose used powerchanges.

[0109] Moreover, when the electric power difference between the marginload power and the outputable power is larger than the inputable poweras a result of the amount judgment, the electric power is generated suchthat the inputable power becomes the charged power of the power storageunit, and the flow rate or the pressure of the gas supplied to the fuelcell is controlled such that the deficient amount of the electric powerobtained by subtracting the inputable power and the outputable powerfrom the margin load power is always generated. Therefore, even if thepower storage unit cannot sufficiently supply the electric power withrespect to the load change due to a low temperature and the like, itbecomes possible to immediately supply the electric power to the loadcircuit.

[0110] Also, according to the control apparatus of the fuel cell systemof the present invention, it is judged that the fuel cell cannotgenerate the electric power equivalent to the deficient power amountwith respect to the required margin power based on at least one of thetemperature of the fuel cell, the stoichiometric ratio, the flow rate,and the pressure of the oxidant gas, and the stoichiometric ratio, theflow rate, and the pressure of the fuel gas. Therefore, it becomespossible to notify that there remains little power left for the loadchange, depending on the state of the fuel cell.

[0111] Furthermore, according to the control apparatus of the fuel cellsystem of the present invention, when the fuel cell cannot generate theelectric power equivalent to the deficient power amount, the user isnotified that the required margin power cannot be secured. Therefore, itbecomes possible to accurately notify that there remains little powerleft for the load change.

[0112] Still furthermore, in the control apparatus of the fuel cellsystem of the present invention, the electric power supplied from thepower storage unit to the load circuit is set to be equal to theoutputable power or smaller, and the electric power inputted from thefuel cell to the power storage unit is set to be equal to the inputablepower of the power storage unit or smaller. Therefore, it becomespossible to inhibit the progression of deterioration of the powerstorage unit.

[0113] According to the control method of the fuel cell system of thepresent invention, when the outputable power is smaller than the marginload power as a result of the amount judgment, the power generationamount of the fuel cell is controlled such that the charged powerbecomes larger than the electric power difference between the marginload power and the outputable power. Therefore, even if the load of theload circuit is increased, the electric power is supplied from the powerstorage unit, and the electric power equivalent to the margin load powercan be supplied. Thus, it is possible to realize sufficient power supplyto the load whose used power changes.

[0114] Moreover, when the electric power difference between the marginload power and the outputable power is larger than the inputable poweras a result of the amount judgment, the electric power is generated suchthat the inputable power becomes the charged power of the power storageunit, and the flow rate or the pressure of the gas supplied to the fuelcell is controlled such that the deficient amount of the electric powerobtained by subtracting the inputable power and the outputable powerfrom the margin load power is always generated. Therefore, even if thepower storage unit cannot sufficiently supply the electric power withrespect to the load change due to a low temperature and the like, itbecomes possible to immediately supply the electric power to the loadcircuit.

[0115] Also, according to the control method of the fuel cell system ofthe present invention, it is judged that the fuel cell cannot generatethe electric power equivalent to the deficient power amount for therequired margin power based on at least one of the temperature of thefuel cell, the stoichiometric ratio, the flow rate, and the pressure ofthe oxidant gas, and the stoichiometric ratio, the flow rate, and thepressure of the fuel gas. Therefore, it becomes possible to notify thatthere remains little power left for the load change in accordance withthe state of the fuel cell.

[0116] Furthermore, according to the control method of the fuel cellsystem of the present invention, when the fuel cell cannot generate theelectric power equivalent to the deficient power amount, the user isnotified that the required margin power cannot be secured. Therefore, itbecomes possible to accurately notify the user that there remains littlepower left for the load change.

[0117] Still furthermore, in the control method of the fuel cell systemof the present invention, the control means sets the electric powersupplied from the power storage unit to the load circuit to be equal tothe outputable power or smaller, and also sets the electric powerinputted from the fuel cell to the power storage unit to be equal toinputable power of the power storage unit or smaller. Therefore, itbecomes possible to inhibit the progression of deterioration of thepower storage unit.

[0118] Although the invention has been described above by reference tocertain embodiments of the invention, the invention is not limited tothe embodiments described above, and alterations and modifications willoccur to these skilled in the art, in light of the teachings. The scopeof the invention is defined with reference to the following claims.

1. A control apparatus for a fuel cell system, in which fuel gas andoxidant gas are supplied to a fuel cell to generate electric power, aload is driven by supplying the generated power to a load circuit, andthe generated power from the fuel cell is stored in a power storageunit, comprising: a margin load power setting section for setting amargin load power amount, which is an increased amount of the electricpower supplied from said power storage unit and said fuel cell to theload circuit when said load of said load circuit is increased at apredetermined rate; an outputable power computing section for computingoutputable power of said power storage unit; an inputable powercomputing section for computing electric power capable of being inputtedand stored in said power storage unit; and a control section forcomparing the margin load power set by said margin load power settingsection and the outputable power computed by said outputable powercomputing section to generate an amount judgment result, comparing saidinputable power and an electric power difference between said marginload power and said outputable power when said outputable power issmaller than the margin load power as a result of said amount judgmentto generate an amount judgment result, generating electric power to setsaid inputable power to be charged power of said power storage unit whenelectric power difference between said margin load power and saidoutputable power is larger than said inputable power as a result of saidamount judgment, and controlling a flow rate or pressure of gas suppliedto said fuel cell to make it possible to always generate electric powerequivalent to deficient amount of electric power obtained by subtractingsaid inputable power and said outputable power from said margin loadpower.
 2. The control apparatus for the fuel cell system according toclaim 1, further comprising: a section for judging that said fuel cellcannot generate the electric power equivalent to the deficient poweramount for said required margin power based on at least one of thetemperature of the fuel cell, the stoichiometric ratio, the flow rate,and the pressure of the oxidant gas, and the stoichiometric ratio, theflow rate, and the pressure of the fuel gas.
 3. The control apparatusfor the fuel cell system according to claim 1, further comprising: anotification section for notifying a user that said margin load powercannot be secured when said fuel cell cannot generate the electric powerequivalent to said deficient power amount.
 4. The control apparatus forthe fuel cell system according to claim 1, wherein said control sectionsets the electric power supplied from said power storage unit to theload circuit to be equal to said outputable power or smaller, and alsosets the electric power inputted from said fuel cell to said powerstorage unit to be equal to said inputable power of said power storageunit or smaller.
 5. A control method for a fuel cell system, in whichfuel gas and oxidant gas are supplied to a fuel cell to generateelectric power, a load is driven by supplying the generated power to aload circuit, and the generated power from the fuel cell is stored in apower storage unit, comprising the steps of: setting in advance a marginload power amount, which is an increased amount of an electric poweramount supplied from said power storage unit and said fuel cell to theload when said load of said load circuit is increased at a predeterminedrate; computing electric power capable of being inputted and stored insaid power storage unit, and computing outputable power of said powerstorage unit; generating an amount judgment result by comparing saidinputable power and electric power difference between said margin loadpower and said outputable power; and generating the electric power suchthat said inputable power becomes the charged power to the power storageunit, and controlling the flow rate or the pressure of the gas suppliedto said fuel cell such that a deficient amount of the electric powerobtained by subtracting said inputable power and said outputable powerfrom said margin load power is always generated when the electric powerdifference between said margin load power and said outputable power islarger than the inputable power as a result of said amount judgment. 6.The control method of the fuel cell system according to claim 5, whereinit is judged that said fuel cell cannot generate the electric powerequivalent to the deficient power amount for said required margin powerbased on at least one of the temperature of said fuel cell, astoichiometric ratio, a flow rate, and pressure of the oxidant gas, anda stoichiometric ratio, a flow rate, and pressure of the fuel gas. 7.The control method of the fuel cell system according to claim 5, whereina user is notified that the required margin power cannot be secured whensaid fuel cell cannot generate the electric power equivalent to saiddeficient power amount.
 8. The control method of the fuel cell systemaccording to claim 5, wherein the electric power supplied from saidpower storage unit to said load circuit is set to be equal to theoutputable power or smaller, and also the electric power inputted fromsaid fuel cell to said power storage unit is set to be equal to saidinputable power of said power storage unit or smaller.
 9. A controlapparatus for a fuel cell system, in which fuel gas and oxidant gas aresupplied to a fuel cell to generate electric power, a load is driven bysupplying the generated power to the load circuit, and the generatedpower from the fuel cell is stored in a power storage unit, comprising:margin load power setting means for setting a margin load power amount,which is an increased amount of the electric power supplied from saidpower storage unit and said fuel cell to a load circuit when said loadof said load circuit is increased at a predetermined rate; outputablepower computing means for computing outputable power of said powerstorage unit; inputable power computing means for computing electricpower capable of being inputted and stored in said power storage unit;and control means for comparing the margin load power set by said marginload power setting means and the outputable power computed by saidoutputable power computing means to generate an amount judgment result,comparing said inputable power and an electric power difference betweensaid margin load power and said outputable power when said outputablepower is smaller than the margin load power as a result of said amountjudgment to generate an amount judgment result, generating electricpower to set said inputable power to be charged power of said powerstorage unit when an electric power difference between said margin loadpower and said outputable power is larger than said inputable power as aresult of said amount judgment, and controlling a flow rate or pressureof gas supplied to said fuel cell to make it possible to always generateelectric power equivalent to deficient amount of electric power obtainedby subtracting said inputable power and said outputable power from saidmargin load power.